Electronic telephone line interface circuits are known to provide battery feed, loop closure detection, and other functions. Lines requiring loop closure detection are sometimes called loop-start circuits because a telephone call initiated at a subscriber station by going off-hook causes the subscriber station to present a closed circuit to direct current. That condition is detected, e.g., in a serving local telephone switching office as an indication that the subscriber is initiating a call. Loop closure detection is subsequently employed in the local office to detect dialing pulses transmitted from a subscriber station. An example of this type of electronic line feed arrangement on a loop-start circuit is found in the copending application, Ser. No. 346,585, now U.S. Pat. No. 4,476,350, filed Feb. 8, 1982, of D. W. Aull and D. A. Spires, entitled "Battery Feed Circuit", and assigned to the same assignee as the present application. A U.S. Pat. No. 4,283,604 to C. W. Chambers, Jr. is another example of such an electronic line feed and interface circuit.
In telephone systems, there are also line circuits, e.g., those between a central office and certain private branch exchange (PBX) arrangements, wherein a call is initiated by applying ground to one conductor, e.g., the ring conductor, of a two-wire loop circuit when a call is initiated. This is called a ground-start circuit. The presence of ground on such a conductor, which had previously been electrically floating, is sensed at the central office as a signal of the subscriber action. Such arrangements are not directly useful in systems normally dependent on utilization of a loop-start mode of operation, and adapting interfaces are employed to accomplish translation between loop-start and groundstart. Examples of such translation circuits are a U.S. Pat. No. 3,721,768 to F. P. Mazac et al. and a U.S. Pat. No. 4,221,936 to B. J. N. Vaughan. It is also known alternatively, to employ a special ground-start line interface unit when waiting for call initiation; and, thereafter, to employ a loop-start line interface unit when awaiting dialing pulses.
In channel path treatment circuits in channel units, it is often necessary that certain signal functions be bypassed around the treatment circuits. It is, then, necessary to provide a line feed interface at points where the signaling bypass path is separated from the regular channel path. If such an interface is in a channel serving a PBX and faces the PBX, the interface must be adapted to accommodate both loop-start and ground-start functions.